unparsed_uri setting in ngx_http_subrequest?

[Brian Pane]

> Headers which values evaluate to empty string aren't sent.

Thanks, that is the missing piece that made the problem much easier.
For some of the headers I need to conditionally set, there is no safe
default value.  But since the header is not sent if the value is an
empty string, I can use proxy_set_header plus variables.  That's a lot
simpler than the workaround I was going to use (building a new
headers_in structure in the subrequest).

-Brian